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The spacecraft was still capable of operating with just three of the wheels, but it lost another in May and attempts to fix either of them have failed.

Scientists are confident, however, that the telescope can conduct a different type of scientific programme using the two remaining reaction wheels and thrusters.

John Grunsfeld, associate administrator for NASA’s science mission directorate, said: “Kepler has made extraordinary discoveries in finding exoplanets including several super-Earths in the habitable zone.

“Knowing that Kepler has successfully collected all the data from its prime mission, I am confident that more amazing discoveries are on the horizon.”

Using the data collected from the first half of the Kepler mission, scientists have already confirmed the existence of 135 exoplanets and identified more than 3,500 candidates. Hundreds more discoveries are expected as more data are analysed.

William Borucki, Kepler science principal investigator, said: “At the beginning of our mission, no one knew if Earth-size planets were abundant in the galaxy. If they were rare, we might be alone.

"Now at the completion of Kepler observations, the data holds the answer to the question that inspired the mission: Are Earths in the habitable zone of stars like our Sun common or rare?”